1. Field of the Invention
The present invention relates to digital signal processing and receiving apparatuses and, more particularly, to a digital signal processing and receiving apparatus, which separates a code signal according to a modulation signal from an analog reception signal received by a reception antenna.
2. Description of the Related Art
Conventionally, as disclosed in, for example, Japanese Laid-Open Patent Application No. 8-8744, a signal processing apparatus, which applies a digital signal process to a reception signal after applying an A/D conversion process. In this signal processing apparatus, a reception signal is sample-held, and an A/D process is repeated continuously for N times during the sample-hold. Then, the N sets of digital data are subjected to a spectrum analysis according to a Fourier transformation so as to take out a sample signal having only a direct-current component. Therefore, according to the above-mentioned conventional signal processing apparatus, a frequency component other than a direct-current component can be removed from a reception signal, and, as a result, a noise of a high-frequency component equal to or higher than a sampling frequency can be eliminated.
On the other hand, there is known an in-vehicle equipment remote control system that performs a control of permission or non-permission of use of a vehicle in accordance with a result of a code check according to radio communication between a portable equipment and an in-vehicle equipment. This system is a system, which, when it is determined that contents of a code according to a modulation signal transmitted from a portable equipment by a predetermined carrier wave in an in-vehicle equipment, locks or unlocks a vehicle door and permits a start of an engine or an electric motor as a power source or checks whether or not the portable equipment exists in the vehicle after start of the vehicle. If the above-mentioned conventional signal processing apparatus is used when extracting contents of a cord from the modulation signal transmitted by the portable equipment in the in-vehicle remote control system, the code contents can be extracted while eliminating a noise of a high-frequency component equal to or higher than a sampling frequency.
In the above-mentioned in-vehicle remote control system, the communication between the in-vehicle equipment and the portable equipment is performed even in a state where a control unit, which controls an engine or an electric motor, is in operation or a state where the engine or the electric motor is being driven. For this reason, there may be a case in which an interference wave is superposed on a frequency band of a carrier wave in a communication between the in-vehicle equipment and the portable equipment, the interference wave being a narrow-band noise such as a clock dividing wave caused by a start of a control unit and a wide-band noise such as an ignition noise, a motor noise, an inerter noise, etc. If such an interference wave is a noise having a large energy, a communication error tends to occur in a communication between the in-vehicle equipment and the portable equipment. Therefore, in order to always perform a communication between the in-vehicle equipment and the portable equipment, it is necessary to surely eliminate an interference wave having a large energy superposed on a frequency band of a carrier wave.
However, it is impossible for the above-mentioned conventional signal processing apparatus to eliminate all interference waves due to various noises that can be generated in a vehicle. For this reason, even if the above-mentioned conventional signal processing apparatus is applied to the above-mentioned in-vehicle remote control system, contents of a code sent from a potable equipment with a carrier wave cannot be demodulated appropriately, which may result in that a check of the contents of the code is not performed appropriately.